1. Field of the Invention
The present invention relates to polarization optical systems and projection-type liquid-crystal display devices using the systems.
2. Description of the Related Art
In a conventional projection-type liquid-crystal display device, by reducing, when light is shielded, light transmittance for light flux incident on the center of a liquid-crystal light valve, contrast improvement has been achieved. However, when the light in the periphery of the liquid-crystal light valve is utilized, the light shielding efficiency is deteriorated; consequently, a problem has been that the contrast of the system is decreased.
Accordingly, a liquid-crystal display device has been proposed in Japanese Laid-Open Patent Publication No. 2001-42314 (refer to paragraph 0022, FIG. 5, and FIG. 28), which is provided with a polarization optical system including a first polarization plate, arranged at the incident side of a liquid-crystal light valve, for transmitting first linearly-polarized light, and a second polarization plate, arranged at the light exit side of the liquid-crystal light valve, for transmitting second linearly-polarized light whose polarizing axis is different from that of the first linearly-polarized light, so that the liquid-crystal-light-valve peripheral-light shielding efficiency can be improved. Moreover, in the above patent document, a configuration has been also proposed, in which a phase difference film, rotated to tilt at a predetermined angle around the polarization axis of the first or the second polarization plate as the axis of rotation, is further arranged, and the angular difference between the polarizing axis of the second polarization plate and a pretilt angle of a liquid-crystal molecule used for the liquid-crystal light valve is compensated, so that transmitted light not having been modulated by the liquid-crystal light valve is effectively absorbed by the second polarization plate.
However, light flux having an angular distribution around its optical axis is emitted from the liquid-crystal light valve. Therefore, even though the plane polarization plate and the phase difference film arranged with a tilt angle is used as described above, a part of the light flux incident at a predetermined angle, out of emitted light beams not having been modulated by the liquid-crystal light valve, is not completely absorbed by the polarization plate arranged at the light exit side of the liquid-crystal light valve; consequently, a problem has been that the contrast of the system is decreased.